The physical processes influencing morphodynamics in braided rivers

A case study on the Ayeyarwady River

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Abstract

The Ayeyarwady River is a river with large hydrological variations and an abundance of relatively fine non-cohesive sediment, causing the dynamic behaviour of the river with fast shifting channels. This dynamic behaviour, together with the large hydrological variations, is the source of a frequently flooding river with a moderate navigability. In order to reduce flood risk and to improve navigability, river training measures and sediment management are needed. For an effective and sustainable implementation of these measures, one has to be able to predict the effects of these measures. For braided rivers, of which their behaviour is regarded as complex and hard to predict, this still remains challenging. It requires a good understanding of all the processes playing a role in the morphodynamics of this braided river. The goal of this study was to explore the most dominant of these processes using both satellite imagery and the 2D depth-averaged model Delft3D and to test the ability of the 2D depth-averaged model to simulate the morphodynamics in braided rivers. The main physical processes were found from literature. The physical processes were investigated in an analysis of the system (mainly with satellite imagery), with a sensitivity analysis and with a comparison between model simulations and satellite imagery. The literature study showed that the many processes are related in different ways and all affect the morphology. Since the morphology also influences these processes, a positive feedback exists in which change in one process causes change in the other processes. With large hydrological variations and a large variability in sediment discharge, morphodynamic change can continuously be found. The main driving factor for the dynamics turned out to be the sediment discharge at the upstream end, where vegetation plays a dominant role in the decrease of the dynamics. The actual influence on the dynamics of bank erosion by failure still has to be investigated in more detail, since this was not included in the model. Other processes have minor effect on the dynamics, but do play an important role in how the morphology will develop. This means that they influence widths and depths of the channels and sizes and shapes of the bars. The most important of these processes are the hydrological variations, the sediment transport by primary flow and the deflection of sediment transport by spiral flow and bed-slope effects. With the uncertainty in the sediment discharge and the absence in the model of the processes vegetation and bank erosion by failure, it is impossible make exact predict with the model. The model showed, however, that it is able to simulate local morphodynamics. For engineering purposes, this means the model cannot be used to assess the time and location of erosion and deposition, for that it can be used to asses risk areas for erosion and deposition. In this way it can be used for impact assessment of different human interventions as river training works and sediment management.